1. Field of the Invention
The present invention pertains to the field of DC power supplies. In particular, the present invention pertains to a DC power supply with an output voltage discharge circuit for providing a bias voltage to a liquid crystal display (LCD) panel.
2. Background Information
The present invention relates to a DC power supply with an output voltage discharge circuit. The DC power supply of the present invention may be advantageously used to power an LCD video panel for various electronic devices, such as notebook computers. LCD panels are used in various applications, primarily because they are smaller and consume less energy than a cathode ray tube (CRT) video display. As a result, LCD panels are typically used in portable devices. An LCD panel typically requires four logic and control signals and two power signals for operation. A frame panel clock, a line clock, and a pixel clock control the timing of the LCD panel. A logic signal controls the illumination status at each pixel location on the LCD panel. The LCD panel typically must also be provided with a contrast power signal and a backlight power signal.
LCD panels typically consume substantial amounts of electrical energy. Thus, particularly for battery-powered devices, the LCD panel can be advantageously turned off when not in use to conserve power. An LCD panel can be turned off by removing the logic and control signals, along with the power signals. However, the specifications for several commercial LCD panels indicate that the contrast power signal should be shut off within about 15 milliseconds of the time that the logic and control signals are disabled. Otherwise, the useful life of the LCD panel can be considerably shortened.
It is simple enough to disable the power supply that provides the contrast power signal within 15 milliseconds of the time that the logic and control signals are disabled. However, the voltage at the output of the power supply may remain at too large a value for much longer than 15 milliseconds. Power supplies typically have a capacitor at the output to filter the output voltage. When the power supply is turned on, the output capacitor accumulates a charge. When the power supply is turned off, the output capacitor typically discharges through the LCD panel, which typically takes approximately 90 milliseconds. Thus, the output voltage from the power supply is applied to the input of the LCD panel for a much longer period of time than the specified 15 millisecond maximum time limit.
Some manufacturers have placed a switch, such as a transistor, between the output capacitor of the power supply and the LCD panel. When the contrast power supply is enabled, a control signal closes the switch to allow the power supply to provide contrast power to the LCD panel. When the contrast power supply is disabled, a control signal opens the switch to remove the output voltage from the LCD panel. While this design can remove the output voltage within the specified 15 milliseconds, this design is undesirable for two reasons. First, the placement of the transistor between the power supply and the LCD panel causes a large amount of current to flow through the transistor, which dissipates a considerable amount of electrical energy. Second, a relatively expensive transistor must be chosen to reduce the resistance at this critical point.